1. Field of the Invention
The present invention generally relates to regenerative expansion apparatuses, pulse tube cryogenic coolers, magnetic resonance imaging apparatuses, nuclear magnetic resonance apparatuses, superconducting quantum interference device flux meters, and magnetic shielding methods of the regenerative expansion apparatuses.
More specifically, the present invention relates to a regenerative expansion apparatus configured to shield the magnetic field change due to a magnetic cold storage material, a pulse tube cryogenic cooler, a magnetic resonance imaging apparatus, a nuclear magnetic resonance apparatus, a superconducting quantum interference device flux meter, and a magnetic shielding method of the regenerative expansion apparatus.
2. Description of the Related Art
In recent years, in a system having a superconducting magnet such as a Magnetic Resonance Imaging (MRI) apparatus, a cryogenic cooler has been used so that the superconducting magnet is cooled at a cryogenic temperature. As the cryogenic cooler, for example, a cooler such as a Gifford McMahon (GM) cryogenic cooler or a pulse tube cryogenic cooler has been used. These coolers are regenerative cryogenic coolers configured to perform adiabatic expansion of coolant gas so that cryogenic cooling is generated at that time, and to regenerate the cryogenic cooling of a cold storage material.
The regenerative cryogenic cooler includes a regenerative expansion apparatus and a compressor. The regenerative expansion apparatus is configured to regenerate cryogenic cooling, the cryogenic cooling being generated at the time of the adiabatic expansion of coolant gas. The compressor is configured to receive the coolant gas from the regenerative expansion apparatus, compress the received coolant gas, and resupply the compressed coolant gas to the regenerative expansion apparatus.
The regenerative expansion apparatus includes a cold storage material to be cryogenically cooled. It is necessary for the cold storage material to have a high specific heat at a cryogenic temperature which is the temperature to be used. However, in general, the specific heat of a metal such as lead drastically decreases at cryogenic temperatures equal to or lower than 15 K as the temperature is decreased. Inversely, the specific heat of helium being used as the coolant gas increases as the temperature is decreased.
Accordingly, it is not possible to sufficiently transfer heat from the cold storage material by helium gas cooled by adiabatic expansion. Hence, the temperature of the regenerative expansion apparatus cannot reach the cryogenic temperature such as 4.2 K.
Because of this, in order to make the temperature of the regenerative expansion apparatus reach the cryogenic temperature, a magnetic cold storage material such as HoCu2 is used as the cold storage material. This magnetic cold storage material has a specific heat greater than that of lead at a temperature equal to or less than 15 K. At the temperature equal to or less than 15 K, the magnetic cold storage material undergoes phase transition so that the magnetic cold storage material becomes antiferromagnetic. The antiferromagnetic magnetic cold storage material compared to lead or the like has a large magnetic susceptibility.
Therefore, if the magnetic cold storage material moves in a magnetic field such as one accompanying operations of the regenerative cryogenic cooler, the magnetic field changes in the vicinity of the regenerative cryogenic cooler. For example, in a case where the regenerative cryogenic cooler is arranged close to the superconducting magnet, the magnetic cold storage material included in the regenerative cryogenic cooler is magnetized due to the magnetic field generated by the superconducting magnet and the magnetized magnetic cold storage material moves in the magnetic field generated by the superconducting magnet. Therefore, the magnetic field generated by the superconducting magnet is disarranged.
Because of this, the following method has been suggested. That is, the magnetic field generated by the magnetic cold storage material included in the regenerative expansion apparatus is shielded by using a superconducting magnetic shield member.
An example of a GM cryogenic cooler where an alloy or a chemical compound using a rare earth metal is used as the magnetic cold storage material and a magnetic shield member made of a superconductor is provided in the periphery of the magnetic cold storage material is described in Japanese Laid-Open Patent Application Publication No. 2-161260. In this example, the magnetic cold storage material fills a displacer which is a movable part of the GM cryogenic cooler. A superconducting magnetic shield part is provided in the periphery of the displacer that the magnetic cold storage material fills.
On the other hand, the pulse tube cryogenic cooler does not include a movable part such as the displacer of the GM cryogenic cooler. Therefore, in the pulse tube cryogenic cooler compared to other coolers such as the GM cryogenic cooler, the moving distance of the magnetic cold storage material is short and the disturbance of the magnetic field is small. Because of this, the pulse tube cryogenic cooler is advantageous in a system having a superconducting magnet.
However, in a case where the pulse tube cryogenic cooler is used for a micro-magnetic field measuring system configured for a micro-magnetic field, such as for MRI, even the micro-magnetic field change due to micro-vibration of the pulse tube cryogenic cooler causes disturbance of the magnetic field. Accordingly, in the case where the pulse tube cryogenic cooler is used for the micro-magnetic field measuring system, a superconducting magnetic shield member is provided for the pulse tube cryogenic cooler.
An example where the superconducting magnetic shield member is provided for the pulse tube cryogenic cooler is discussed with reference to FIG. 1. Here, FIG. 1 is a schematic view of a related art regenerative expansion apparatus for the pulse tube cryogenic cooler.
As shown in FIG. 1, a regenerative expansion apparatus 110 includes a first regenerative tube 111, a second regenerative tube 112, a first pulse tube 113, a second pulse tube 114, a first stage cooling stage 115, a second stage cooling stage 116, a flange 117, a valve unit 118, a cold storage material 122, a magnetic cold storage material 123, and a superconducting magnetic shield member 127.
The flange 117 is an original point (starting point) of a high temperature side. The first stage cooling stage 115 and the second stage cooling stage 116 are provided with separation. The first regenerative tube 111 and the first pulse tube 113 are provided between the flange 117 and the first stage cooling stage 115. The second regenerative tube 112 is provided between the first stage cooling stage 115 and the second stage cooling stage 116. The second pulse tube 114 is provided between the flange 117 and the second stage cooling stage 116.
The cold storage material 122 and the magnetic cold storage material 123 fill, in this order, from the first stage cold stage 115 to the second regenerative tube 112 along a direction to the second stage cold stage 116. The superconducting magnetic shield member 127 surrounds a part of the second regenerative tube 112 that the magnetic cold storage material 123 fills and thermally comes in contact with the second stage cooling stage 116. The lower end of the superconducting magnetic shield member 127 is fixed to the second stage cooling stage 116 by solder.
The temperature of the first stage cooling stage 115 is maintained at approximately 20 K through approximately 100 K. The temperature of the second stage cooling stage 116 is maintained at approximately 4 K through approximately 10 K. Since the lower end of the superconducting magnetic shield member 127 comes in thermal contact with the second stage cooling stage 116, the temperature of the superconducting magnetic shield member 127 is maintained substantially the same as the temperature of the second stage cooling stage 116.
In addition, in a case where the regenerative expansion apparatus 110 cools liquid helium in a low temperature vessel, a part below the flange 117 is filled with helium gas where the liquid helium is evaporated.
In the meantime, an example of a pulse tube cryogenic cooler having a regenerative expansion apparatus having a superconducting magnetic shield member is described in Japanese Laid-Open Patent Application Publication No. 2007-155319.
In addition, an example of a pulse tube cryogenic cooler provided in a superconducting magnetic system, where the pulse tube cryogenic cooler is filled with the magnetic cold storage material, is described in Japanese Laid-Open Patent Application Publication No. 2006-38446. In this pulse tube cryogenic cooler, a superconducting magnetic shield member is provided so that a stray magnetic field due to the magnetic cold storage material is reduced.
However, in the case of a magnetic field change due to the magnetic cold storage material of a regenerative cryogenic cooler having the regenerative expansion apparatus where the cold storage material is shielded by using the superconducting magnetic shield member, the following problems may occur.
In any methods described in Japanese Laid-Open Patent Application Publication No. 2-161260, Japanese Laid-Open Patent Application Publication No. 2007-155319, and Japanese Laid-Open Patent Application Publication No. 2006-38446, in order to maintain a superconducting state of the superconducting magnetic shield member, the superconducting magnetic shield member has to be cooled at a temperature equal to or lower than a superconducting critical temperature.
However, as shown in FIG. 1, for example, the second regenerative tube 122, filled by the magnetic cold storage material 123 where the superconducting magnetic shield member 127 shields the magnetic field, is arranged between the first stage cold stage 115 and the second stage cold stage 116. Accordingly, the second regenerative tube 112 is in a range whose temperatures are approximately 4 K through approximately 10 K. On the other hand, helium gas is situated between the superconducting magnetic shield member 127 and the second regenerative tube 112. Hence, the temperature of the superconducting magnetic shield member 127 increases due to heat transfer via helium gas so that the temperature of the second stage cold stage 116 which thermally comes in contact with the superconducting magnetic shield member 127 increases.
As a result of this, the cryogenic cooling capacity of the cryogenic cooler is degraded so that a designated cryogenic temperature cannot be achieved.